Field of the Invention
The present invention relates to an igniter that controls an ignition coil connected to a spark plug of an engine.
Description of the Related Art
FIG. 1 is a perspective view of an engine room 101 provided to a gasoline-engine vehicle (which will also be referred to simply as the “vehicle” hereafter) 100. The engine room 101 houses an engine 110, an intake manifold 112, an air cleaner 113, a radiator 114, a battery 102, and the like. FIG. 1 shows a four-cylinder engine.
The engine 110 is provided with a plug hole (not shown) for each cylinder. A spark plug (not shown) is inserted into each plug hole. Each cylinder of the engine 110 receives a supply of a mixture of air transmitted via the air cleaner 113 and the intake manifold 112 and a fuel gas supplied from an unshown fuel tank. Each spark plug is ignited (a spark is generated) at an appropriate timing, so as to start rotational driving of the engine.
FIG. 2 is a block diagram showing a part of an electrical system of a vehicle 100r. The vehicle 100r includes a battery 102, an ignition coil 104, a spark plug 106, an ECU 108, and an igniter 200. The ECU 108 generates an ignition signal IGT, which indicates an ignition timing for the spark plug 106, in a cyclic manner in synchronization with the rotation of the engine 110. A secondary coil L2 of the ignition coil 104 is connected to the spark plug 106. The igniter 200 controls the current that flows through a primary coil L1 of the ignition coil 104 according to the ignition signal IGT, so as to generate a high voltage (secondary voltage VS) of several tens of kV at the secondary coil L2. This provides a discharge of the spark plug 106, thereby providing combustion of the mixture gas stored in the engine 110.
The igniter 200 includes a switch element 202 and a switch control apparatus 300r. The switch element 202 is configured as an IGBT (Insulated Gate Bipolar Transistor) arranged such that its collector is connected to the primary coil L1 and its emitter is grounded. The switch control apparatus 300r controls the voltage at the control terminal (gate) of the switch element 202 according to the ignition signal IGT, so as to control the on/off operation of the switch element 202. Specifically, during a period in which the ignition signal IGT is set to high level, the switch element 202 is turned on. When the switch element 202 is turned on, a battery voltage VBAT is applied between both ends of the primary coil L1. In this state, a current that flows through the primary coil L1 rises with time. When the ignition signal IGT is switched to low level, the switch control apparatus 300r immediately turns off the switch element 202, which cuts off the current IL1 that flows through the primary coil L1. In this stage, the primary coil L1 generates a primary voltage VL1 (=L·dIL1/dt) of several hundreds of V which is proportional to a temporal differentiation of the current IL1. In this state, the coil L2 generates a secondary voltage VS of several tens of kV, which can be calculated by multiplying the primary voltage VL1 by the winding ratio.
The switch control apparatus 300r includes a judgment stage 300A configured as a first stage and a driving stage 300B configured as a second stage. The judgment stage 300A receives the ignition signal IGT from the ECU 108, and judges the level (high level or low level) of the ignition signal IGT. Typically, such an igniter 200 is employed in the engine room. Accordingly, the igniter 200 is exposed to various kinds of surges and noise. In order to suppress a malfunction of the igniter 200 due to high-frequency noise, the judgment stage 300A is provided with a high-frequency filter 303 that removes high-frequency noise superimposed on the ignition signal IGT. A voltage comparator 302 compares the voltage level VFIL of the ignition signal IGT that has passed through the high-frequency filter 303 with a predetermined reference voltage (threshold value) VREF, so as to generate a binary judgment signal SDET that is set to high level or otherwise low level.
The driving stage 300B switches the switch element 202 between the on state and the off state according to the judgment signal SDET. A delay circuit 304 applies a predetermined delay to the judgment signal SDET. The amount of delay is set such that the time difference (delay) between the transition of the ignition signal IGT and the time point at which the spark plug is discharged matches a predetermined value. The pre-driver 306 and the gate driver 308 control the gate voltage of the switch element 202 according to the output of the delay circuit 304.
As a result obtained by investigating the igniter 200r shown in FIG. 2, the present inventors have come to recognize the following problem. FIG. 3 is an operation waveform diagram showing the operation of the igniter 200r shown in FIG. 2.
At the time point t0, the ignition signal IGT is asserted (set to high level) by the ECU 108. This increases the voltage VFIL of the signal to be input to the voltage comparator 302 after it passes through the high-frequency filter 303. In this state, during a period in which VFIL>VREF, the judgment signal SDET is asserted (set to high level). During a period in which the judgment signal SDET is set to high level, the switch element 202 is turned on, which raises the coil current Ic.
At the time point t1, the ignition signal IGT is negated (set to low level) by the ECU 108. The judgment signal SDET is switched to low level according to the negation, which turns off (cuts off) the switch element 202. In this state, the large voltage VS generated by the secondary coil L2 of the ignition coil 104 is applied to the spark plug 106, thereby providing ignition.
When the ignition signal IGT is switched to low level, the voltage VFIL, which has passed through the filter and which is to be input to the voltage comparator 302, drops with a delay due to the capacitance of the high-frequency filter 303. Spark noise (cut-off noise) occurs due to the ignition of the spark plug 106, which is input to the switch control apparatus 300 via the input terminal IN. If a charge remains in the capacitor of the high-frequency filter 303 at the timing t2 at which spark noise occurs, the input voltage VFIL of the voltage comparator 302 exceeds the reference voltage VREF, which asserts the judgment signal SDET, leading to cut-off occurring again even if the ignition signal IGT remains at low level. The above-described problem is by no means within the scope of common and general knowledge in the field of the present invention. Furthermore, it can be said that this problem has been uniquely recognized by the present inventor.